The present invention relates to a shift arrangement for motor vehicle transmissions and, more particularly, to an improved sector plate incorporated into a transfer case shift mechanism. The sector plate has a modified cam profile adapted to reduce operator shift effort when shifting from a two-wheel drive operating mode to a four-wheel drive operating mode during cold weather conditions.
In general, power transfer mechanisms, such as transfer cases, are operatively associated with both manual and automatic transmissions for selectively directing power to the non-driven wheels of a motor vehicle upon shifting from the two-wheel drive mode to the four-wheel drive mode. More particularly, most conventional transfer cases use a transmission type synchronizer clutch arrangement incorporated with a shift fork mechanism to provide "shift-on-the-fly" two-wheel drive to four-wheel drive mode shifting. When shifting the transfer case between the two-wheel and four-wheel drive operating modes, selective movement of a shift lever by the vehicle operator causes corresponding rotation of a sector plate. A mode pin, secured to the shift fork mechanism slidably mounted on a shift rail, is biased into engagement with a cam edge surface of the sector plate so as to be axially displaced upon rotation of the sector plate. As such, the shift fork mechanism slides on the shift rail for travel between positions defining the two and four-wheel operating modes. Moreover, movement of the shift fork mechanism toward the four-wheel drive position acts to energize the synchronizer clutch apparatus for shifting a clutch sleeve into engagement with the external spline of a silent chain carrier. In this manner, the transfer case is "locked-up" in the four-wheel drive operating mode for delivering power to the vehicle's front and rear axle output shafts.
A common problem associated with many conventional part-time transfer cases is that during cold weather conditions it is often extremely difficult to shift into the four-wheel drive mode. In manually actuated part-time transfer cases, the vehicle operator is required to exert an abnormally large input force on the shift lever to physically overcome the excessive shift resistance associated with "cold" shifting into the four-wheel drive operating mode. This increased shift effort is due primarily to the increased viscosity of the lubricant entrained within the front axle final drive assembly which causes viscous drag loading on the transfer case front output shaft. Under such conditions, the synchronizer clutch apparatus cannot generate sufficient frictional torque loading to effectively complete the speed synchronization process between the front and rear output shafts.
However, this cold weather phenomenon is largely temperature dependent in that the increased shift effort is generally experienced only upon attempting to shift into the four-wheel drive mode immediately after cold starting of the vehicle. Once the vehicle has been in four-wheel drive operation for a short period of time or two-wheel drive operation for a longer period of time, the temperature of the lubricant increases while its viscosity decreases so as to restore relatively normal shifting operation of the synchronizer clutch unit. As will be appreciated, it is undesirable to require the vehicle operator to physically overcome the high shift resistance when manually shifting into the four-wheel drive operating mode. Moreover, under extreme cold temperature conditions it may not only be difficult, but virtually impossible for the vehicle operator to shift the transfer case into the four-wheel drive mode until the fluid temperature has increased above a minimum threshold value.